Devices for the suction filtration of fluids, such as liquid operators, fuels or the like, are used in widely different fields. Due to the increasing requirements related to the emission behavior of internal combustion engines, there has been an increased use more recently of exhaust gas after-treatment systems with which for a selective catalytic reduction of nitrogen oxides, an aqueous urea solution is stored in a tank and is injected into an exhaust gas stream, in order, in conjunction with an SCR catalytic converter, to extract ammonia as the actual reducing agent. The urea solution can be obtained under the trade name “Adblue” at gas stations as an additional operating fluid for motor vehicles, for filling the respective storage tank. High demands are placed on the purity of the urea solution for the operational safety of the associated injector system. Impurities can lead to blockages of channels and/or dosing pumps and/or valves in the injector system.
Because the urea solution provided for filling can contain different impurities, and because, in particular, the danger also exists that during the filling process a quantity of contaminant may be introduced into the tank, filtering is necessary to clean the urea solution which is drawn from the tank.
For this purpose, a device of the aforementioned kind is known from WO 2011/124637 A1, in which the filter element serving as a suction filter is integrated into the tank storing the urea solution. The operating behavior of this device is not optimal for use in motor vehicles. Thus, the safe operation of the system depends on the filling level height in the tank not falling below the upper edge of the filter medium of the filter element. As soon as this is the case, there is the danger that air is suctioned out of the tank via the unused, i.e. exposed, part of the filter medium, and passes into the system. For a useful operation of motor vehicles, however, the filling level height must be able to be lowered down to the bottom area of the tank, in order in this way to remove a large fill quantity from the tank.